Transducers are defined as devices that convert energy or a physical quantity from one form to another. The purpose of a transducer is to generate or detect some signal phenomena. There are basically two wide areas into which transducers are categorized: active and passive. An active transducer generates a signal, typically a voltage or current, as a result of some form of energy or force change such as when a thermocouple generates an electrical signal when heated which is a function of the amount of heat applied; and a passive transducer which changes its properties when exposed to energy. A passive transducer has an element which, under some force, responds by changing its physical properties or behavior. Transduction employs a transfer process that senses or communicates energy or information.
Non-destructive testing (NDT) often uses special transducers to interrogate the internal structure and features of materials that can not be directly observed by ordinary means. As used herein, the term “non-destructive testing” means any testing method which does not involve damaging or destroying the test sample. In many instances the structures or features embedded in the materials are undesirable and are considered flaws or damage. In other instances the internal structures and features are an expected, allowable or necessary component of the material. In either case, obtaining accurate information about the nature and character of these internal structures and features is a fundamental need during the inspection, qualification and diagnosis of these materials. One common transducer in NDT applications is the ultrasound transducer (UT).
In the case of a UT, sound energy is converted to electrical energy or electrical energy is converted to sound energy. In many cases this energy conversion is done through the piezoelectric effect, where an electrical field or potential in a piezoelectric material induces a mechanical stress generating an acoustic field, or conversely, an acoustic field impinging on the piezoelectric material induces an electric field or potential. These piezoelectric materials normally have a complex electrical impedance which requires special methods to integrate them into useful transducers.
Ultrasound is defined as sound whose frequency is above the threshold of hearing (that is, greater than 20 kHz). For practical purposes, signals greater than 100 kHz are typically used. Frequency, along with excitation time width, defines the spatial resolution of the interrogating sound field. A short, high frequency pulse will enable small anomalies and defects in material to be detected. However, with an increase in frequency comes an increase in the attenuation, scattering and absorption of the sound energy. This limits the effective depth that can be probed without too great a loss in signal energy. Consequently, there is an optimal frequency range that trades resolution with received signal response. For ultrasound NDT, a frequency range between 2 and 20 MHz is optimal, with the lower range being most common.
A conventional NDT system configuration is shown in FIG. 1. In this figure an ultrasound system US is connected to an ultrasound transducer (UT) TD via a long coaxial cable CC. The ultrasound system produces a high voltage excitation signal which propagates down the coaxial cable, is applied to a piezoelectric element PE (which includes the piezoelectric material and optional tuning components, indicated as impedance matching and tuning network IN) and generates an acoustic signal from the front of the UT. Received signals are sensed by the UT and sent back down the coaxial cable to the ultrasound system for analysis.
Due to the relative inefficiency of piezoelectric materials used in ultrasound transducers, large excitation signals are often needed to generate a sufficiently intense sound field. In NDT applications, UT excitation signals can be hundreds of volts but normally have a low average power, often less than a Watt. As a receiver, a UT produces extremely small signals, typically on the order of a factor of 1,000 to 1,000,000 down from the excitation signal. This corresponds to a range of input to output amplitudes of 60 to 120 dB.
The very nature of the transduction process involving physical structures tends to distort, delay and degrade the signal being transferred. Distortion of the signal can be caused by nonlinearities, hysteresis, resonances, and environmental effects. Any inadvertent energy storage in the process may cause time delays between the output signal and the event. A time delay is often mathematically modeled as a phase shift. The normal sensing and processing of signals also adds noise, which can degrade the information being transferred. A useful measure of the ability of a system to convey information from a transducer to a measurement or detection unit is the Signal-to-Noise Ratio (SNR). This parameter relates the signal, in whatever dimensions and units are appropriate, to the noise that is present and that would be detected simultaneously with the signal. High SNR values indicate that the signal stands out clearly against the background, whereas low SNR values indicate that the signal is being obscured by the noise. Averaging a number or signal segments in the time or frequency domain may reduce the masking effect of random noise and may improve the SNR.
Although it is possible to increase the intensity of the sound field by increasing the amplitude of the excitation and thus increase the received signal, for practical transducers there is always a physical limit to the amplitude or power that can be applied. Temporary or permanent performance degradation or even failure will be caused to the transducer if the excitation is increased beyond this level.
Additional receiver signal strength can be achieved by applying electronic gain or amplification. However, this approach is an ongoing challenge in ultrasound NDT applications since inappropriately placed amplification may simply increase the noise at the same rate as the signal, which will not produce an improvement in the SNR. Amplification can also introduce other distortion problems such as clipping, saturation effects, dispersion and transition time degradation, which will mask the underlying signal's true character.
The environments where traditional ultrasound NDT techniques are used are often industrial in nature. In these situations there is a large amount of electrical interference from welding equipment, radio communication systems, high power electronics, large motor loads, electrical switching gear or other severe electromagnetic interference (EMI) generators. This noise is often in the same frequency bands as the signal of interest and can enter into the signal path to reduce the SNR. As is known to those skilled in the art, if noise is combined with a signal, it becomes more complex and difficult to extract the original signal. It is advantageous then to increase and maintain the signal strength as soon as possible in the signal path to reduce the relative effects of noise that might be introduced later.
As well, NDT UT's are often used on the distal end of relatively long transmission cables. Transmission cables, or lines, suffer from impedance matching issues, signal attenuation and noise pickup problems.
As used in this disclosure, the term “transmission lines” refer to electrical cables or interconnections whose length is more than a significant fraction (for example, more than 5%-10%) of the characteristic wavelengths of the signal-of-interest being conveyed on the cable or interconnection. As used herein in relation to the length of a signal conductor, the term “short” is taken to mean 5 to 10% of the characteristic wavelength of a signal on the conductor and the term “trivial” is taken to mean less than 1% of the characteristic wavelength whereby a conductor that is short or trivial can essentially be ignored as far as its effect on the signal because its length is short enough to not affect the signal. The term “characteristic wavelengths” refers to the range of wavelengths corresponding to the band of frequencies of interest in the conveyed signal. Non-trivial transmission line impedance matching effects may or may not exist at each of these wavelengths as a result of the physical arrangement of the equipment and the particular construction of the interconnecting conductors.
Other common ultrasound testing configurations include arrangements where two or more approximately equivalent piezoelectric assemblies are used, where a combination of one or more elements are used to produce the acoustic signal and a combination of one or more elements acts as a receiver. This configuration, although more complex in layout, operates on essentially the same transmit-receive basis as the single piezoelectric element configuration. The disclosed invention is equally applicable to these multi piezoelectric element configurations.
The piezoelectric element has a complex impedance behavior. Historically, passive matching networks have been used to prevent impedance mismatch reflections from the piezoelectric element when it is coupled to a transmission line. Reflections can create undesirable signal distortion and introduce excitation artifacts.
Traditionally, the piezoelectric element matching network is formed from a combination of inductive, capacitive and possibly resistive elements that trade signal amplitude for reduced reflection effects. The matching circuitry ensures that the high frequency signal components traveling to and from the transducer are well behaved in the cable, but the matching circuitry does not improve either the power received by the transducer or the power received by the ultrasound system from the transducer.
In addition to the impedance matching network, it is often desired to have a response tuning network that is used to optimize the sensitivity, resonant frequency and bandwidth of the piezoelectric element for a given configuration and application. Often it is not possible to optimize the response of the piezoelectric element at the same time as matching the impedance, as these two functions occur within the same portion of the circuit and are often at odds with each other. There is often a compromise made between matching and tuning that results in a less than ideal response.
When acting as a receiver, the piezoelectric element behaves as an impedance limited voltage source. To recover the largest signal from such a voltage source, it should be coupled to a high input impedance voltage sensing circuit to limit loading effects. In many cases, these circuit requirements and characteristics many not be synonymous with those of the matching circuit for transmission cable matching, so less than ideal received signal transfer occurs when the circuit is only optimized for cable matching.
Signal attenuation can be compensated to some extent by the appropriate addition of gain elements configured and located to prevent undue signal distortion.
Reducing radiated and conducted susceptibility can minimize noise pickup. Antenna effects such as inductive signal path loops and capacitive coupling into circuits may be reduced by proper layout. Circuits may be isolated to break up ground loops and differential signaling techniques can inhibit conducted noise.
High noise environments, long interconnections, lossy matching networks and low signal levels generally combine to degrade the SNR and confound the detection of small, deep or poorly defined structures or features in the material under interrogation.
Therefore, there is a need for a device that can improve the coupling of the UT piezoelectric element to the signal transmission line to preserve the original signal amplitude.
There is also a need to generally increase the signal amplitude with high fidelity early on in the signal path before noise is introduced.
There is also a need to reduce noise coupling or coupling effects.